San Diego Sea Turtles

Monday, January 29, 2018

New habitat data show turtles are spending more time offshore, increasing their risk as fishing bycatch

Populations of green sea turtles living in the eastern region of the Pacific Ocean have rebounded in recent years, but their numbers remain dangerously depleted. Research by led by biologists at the University of California San Diego and NOAA’s National Marine Fisheries Service is offering previously unknown information about where these turtles live and how they use their habitats, key data that will aid future conservation efforts.

Cali Turner Tomaszewicz, a former graduate student and now a postdoctoral researcher in Associate Professor Carolyn Kurle’s laboratory, led the study with new technologies developed to extract life history information from sea turtle bones.

Cali Turner Tomaszewicz inspects a green sea turtle bone.

East Pacific green sea turtles are born in Mexico and spend their lives in the waters along Mexico and the United States, including off the coast of San Diego and in San Diego Bay. Using a technique recently developed by Turner Tomaszewicz, the researchers analyzed the bones of dead green sea turtles washed ashore along the Baja California peninsula. The approach combines bone dating, or “skeletochronology,” and the sequential sampling of annual growth rings for chemical signatures (stable carbon and nitrogen isotope ratios). The signatures in the growth rings revealed the animal’s foraging habits through its life span, not unlike extracting the life history from tree rings. These and complimentary techniques allowed the researchers to reconstruct a multi-year analysis of past green sea turtle residency patterns, demographics and ecological information.

The results, published in the journal Marine Ecology Progress Series, answer vital, previously unknown aspects of the green sea turtles’ habitats, including their age when they move into nearshore habitats, when they start breeding and whether they use nearshore or deeper habitats for feeding. According to the researchers, the new insights about the life-history of green sea turtles in this region have strong applications to the conservation, management and ongoing recovery of this endangered population.

The analysis shows that these green sea turtles spend approximately three to five years as juveniles in the open ocean before settling in nearshore habitats. Smaller turtles at the population’s mainland nesting sites mature and begin breeding at 17 years of age, while larger turtles nesting on the island rookeries begin breeding at 30 years.

“This is the first empirical evidence showing that turtles in this population mature at very different rates at the two distinct nesting sites,” Turner Tomaszewicz added. “This could really affect the ability of this population to recover because the nesting site with turtles that mature slower (30 years) will take much longer to begin contributing to the population increase and recovery.”

Most importantly, the data revealed that these turtles spend most of their time eating in the offshore waters of the Gulf of Ulloa, directly adjacent to the beach where they were found dead. The finding, a surprise to the researchers, carries critical conservation implications since the turtles’ cause of death is presumed to be from being caught in nets (bycatch) from fishing boats in the Gulf of Ulloa.

“The results of our work show that some green turtles, not just loggerhead turtles, are using this habitat for many years at a time and therefore are also at risk of bycatch and in need of protection,” said Turner Tomaszewicz. “Fortunately, much work has been done in this region to reduce these bycatch deaths.”

Cali Turner Tomaszewicz in Baja California with an East Pacific green sea turtle.

“This more carnivorous diet likely helps these turtles grow faster and mature sooner, yet also comes with the tradeoff for being more at risk of becoming fisheries bycatch,” said Turner Tomaszewicz.

The researchers previously applied their novel technique to North Pacific loggerhead turtles (Turner Tomaszewicz et al. 2015, 2017) and now intend to apply their methods to investigate other turtle populations throughout the Pacific.

“Our work really highlights the exciting use of marine vertebrates as passive data collectors for ocean scientists. As animals move through and utilize food from different areas within the oceans, the biochemical signals of those habitats are recorded in the body parts of the animals,” said Kurle, a researcher in the Section of Ecology, Behavior and Evolution in the Division of Biological Sciences. “Analyses of those chemical signatures, and matching them with specific time periods, allows us to reconstruct not only the animals’ movement patterns and diet, but also to better understand the biogeochemical processes characterizing various marine environments. These types of data can be really difficult to obtain using conventional methods that require labor and resource-intensive sampling over long time scales, but with our approach the animals are basically collecting data for us as they make their living in the sea.”

Coauthors of the research include Jeffrey Seminoff of NOAA’s Southwest Fisheries Science Center, Larisa Avens and Lisa Goshe of NOAA’s Southeast Fisheries Science Center, Juan Rguez-Baron of the University of North Carolina Wilmington and S. Hoyt Peckham of Stanford University.

Determining location and timing of ontogenetic shifts in the habitat use of highly migratory species, along with possible intrapopulation variation in these shifts, is essential for understanding mechanisms driving alternate life histories and assessing overall population trends. Measuring variations in multi-year habitat-use patterns is especially difficult for remote oceanic species.

To investigate the potential for differential habitat use among migratory marine vertebrates, we measured the naturally occurring stable nitrogen isotope (δ15N) patterns that differentiate distinct ocean regions to create a ‘regional isotope characterization’, analysed the δ15N values from annual bone growth layer rings from dead-stranded animals, and then combined the bone and regional isotope data to track individual animal movement patterns over multiple years.

We used humeri from juvenile North Pacific loggerhead turtles (Caretta caretta), animals that undergo long migrations across the North Pacific Ocean (NPO), using multiple discrete regions as they develop to adulthood. Typical of many migratory marine species, ontogenetic changes in habitat use throughout their decades-long juvenile stage is poorly understood, but each potential habitat has unique foraging opportunities and spatially explicit natural and anthropogenic threats that could affect key life-history parameters.

We found a bimodal size/age distribution in the timing that juveniles underwent an ontogenetic habitat shift from the oceanic central North Pacific (CNP) to the neritic east Pacific region near the Baja California Peninsula (BCP) (42·7 ± 7·2 vs. 68·3 ± 3·4 cm carapace length, 7·5 ± 2·7 vs. 15·6 ± 1·7 years). Important to the survival of this population, these disparate habitats differ considerably in their food availability, energy requirements and threats, and these differences can influence life-history parameters such as growth, survival and future fecundity. This is the first evidence of alternative ontogenetic shifts and habitat-use patterns for juveniles foraging in the eastern NPO.

We combine two techniques, skeletochronology and stable isotope analysis, to reconstruct multi-year habitat-use patterns of a remote migratory species, linked to estimated ages and body sizes of individuals, to reveal variable ontogeny during the juvenile life stage that could drive alternate life histories and that has the potential to illuminate the migration patterns for other species with accretionary tissues.

Wednesday, June 1, 2016

Today, Wednesday June 1st, Sabrina Mashburn will present on the "Southern California's Sea Turtles: Public Perceptions & Conservation". The work being presented is Mashburn's Capstone project, part of her MAS in Marine Biodiversity and Conservation at UCSD's Scripps Institution of Oceanography.

About Mashburn's presentation:"SoCal Sea Turtles aims to investigate people's perceptions of local sea turtles and their conservation. Through surveys, key informant interviews, and conversations with boaters and fishers from San Diego to Long Beach, we have identified a need for more outreach and education about our local sea turtles. This summer, we will continue working on our website, www.SoCalSeaTurtles.org, as well as giving informative presentations at angler's clubs and yacht clubs, in order to increase the public's knowledge of local sea turtles and what people can do to help conserve them."

Full schedule of Capstone projects being presented, including Mashburn's (at 4:50pm PST) is available, and the presentations will also be on live webcast.

Stable isotope analysis (SIA) of skeletal structures is
increasingly used to reconstruct historical marine animal foraging ecology and
life history. Nitrogen isotope values (δ15N) are used to examine
trophic relationships as there is a predictable 3-5‰ increase in δ15N
values with increasing trophic levels. Carbon isotope values (δ13C)
from animal tissues can be used to reconstruct animal movement patterns due to
spatial variation across productivity gradients and geographic regions. In
order to accurately reconstruct sea turtle diet and habitat use through SIA of
bone tissue, potential differences in the isotope values of the two main
components of bone (70% bioapatite and 30% collagen by weight) must be
addressed. The δ13C values from bone collagen reflect diet-based
carbon, whereas the carbon source in inorganic apatite can vary and is
therefore typically removed via acidification prior to SIA. Our research
involves extracting samples from the individual growth layer rings of sea
turtle humerus bones, which yields very small amounts of bone dust (~1.5mg),
far less than is required for traditional SIA collagen preparation. The primary
objective of this study was to test alternative methods of collagen extraction
from small amounts of sea turtle bone. We measured differences in the δ13C
and δ15N values in bulk bone and isolated collagen to determine the
most accurate method for measurement of bone δ13C and δ15N
values. We measured this in two sea turtle species representing three distinct
populations across two ocean basins (North Pacific loggerhead (n=15, CCL
51-73cm), East Pacific green (n=15, CCL 53-73cm), and Northwest Atlantic
loggerhead (n=20, CCL 70-96 cm)). SIA was performed on paired bulk bone and
bone collagen samples that had been acid-treated (0.25M HCl) to remove
inorganic carbon (total n=50). We measured differences in the δ13C
and δ15N values in bulk bone and isolated collagen to determine the
most accurate method for measurement of bone δ13C and δ15N
values. We measured this in two sea turtle species representing three distinct
populations across two ocean basins (North Pacific loggerhead (n=15, CCL
51-73cm), East Pacific green (n=15, CCL 53-73cm), and Northwest Atlantic
loggerhead (n=20, CCL 70-96 cm)). SIA was performed on paired bulk bone and
bone collagen samples that had been acid-treated (0.25M HCl) to remove
inorganic carbon (total n=50). We tested a second method that involved sampling
individual annual growth layers of 10 Pacific turtle bone sections, previously
processed for skeletochronology. Effect of pre-SIA treatment on the δ13C
and δ15N values was tested against ecologically significant
“detection-differences” that are commonly used in migration and foraging
studies (δ13C: 1‰ and δ15N: 3‰).At these
detection differences, neither δ13C nor δ15N were
ecologically significantly affected by either of the pre-SIA treatment methods,
but there was a consistent depletion effect on δ13C.

About Me

Cali is a Postdoctoral Fellow at University of California, San Diego's Ecology program, having received her Ph.D. in Ecology, Behavior
& Evolution. She is a graduate of UCSD's Scripps Institution of Oceanography, where she received her Masters in Marine Biodiversity and Conservation. Her work conducting environmental monitoring in San Diego includes assisting NOAA researchers monitor the population of green sea turtles in the San Diego Bay. The work highlighted on this blog is in partnership with NOAA's Southwest Fisheries Science Center (see links for more information).